If we have a brief look at some of the more recent, highly successful games (Team Fortress 2, Call of Duty 4, World of Warcraft, Sins of a Solar Empire and World in Conflict - games I just happen to own) we can see that there is a possible relationship between their success and their system requirements.
All games mentioned above have very reasonable system requirements and efficient game engines. If you own a system that was built 4 years ago, chances are you could still play those games reasonably well (with some small, expected sacrifices to graphics).
Of course the games mentioned above could have just had excellent designs that appealled to alot of gamers out there, but its a big coincidence that they also have low system requirements.
Yes, let's have a look at those recent, highly successful games you mentioned:
Team Fortress 2's success is an unknown quantity - it was not sold on it's own for a long time after release. There's no hard data on how successful it would have been on it's own. Note also that it was a Valve game, as well as a sequel (thus being part of an already established brand). It's success did not depend on system Reqs.
CoD 4 is, again, a sequel to a highly successful brand, and has been subjected to a long and expensive ad campaign. It's success did not depend on System Reqs.
World of Warcraft, yet again, is part of an already established brand, by a developer that get's a free pass by 99% of the gaming media and their followers. It's success did not depend on System Reqs. Note also that WoW is a statistical outlier that is unrivaled, yet sadly taken as "the goal" for MMO development. 11 Million paying subscribers, 132 million dollars per month, is neither normal, nor necessary for a successful MMO, or any game for that matter.
Sins of a Solar Empire was a "success" due to several factors: The amount of sold units that constitutes "success" in it's genre is far lower than those of the previous examples (incidentally the same is true for WiC). It had heavy support from the enthusiast press, due to it's DRM-less nature, and it's moderate system reqs were bought by skimping on visual detail and restricting modders.
As for World in Conflict - I have yet to see evidence of it's success. It is by now all but forgotten by everyone but's its most hardcore playerbase, with the upcoming expansion being an example of "too little, too late". It's success (if it had that) was not a result of it's low system requirements, although this is one of the games where your claim of efficient game engines is true to an extreme. WiC looks far better than it's hardware need would suggest.
This relationship is somewhat true when viewed in the opposite sense. Crysis while being a solid, good quality title with revolutionary graphics (I happen to enjoy it alot and am fortunate enough to be able to enjoy it at high settings) - has very demanding system requirements. This may have had the effect of limiting the amount of sales the title should've really deserved, because no-one is going to buy a game they can't run at a reasonable level.
Crysis being considered a failure is largely the result of an enormous failure of their PR/Marketing. Someone, somewhere started the Urban Myth that Crysis needs a NASA supercomputer to run and all but a scant few press outlets simply parroted that claim without factchecking it. After they propagated it, the potential buyers believed it too, again without factchecking it.
The truth is Crysis is actually very well optimized. It runs with maxed out settings on a system that is cheaper or as expensive as a run of the mill "next-gen" console.